1. Field of the Invention
The present invention relates to a process for the production of a safety glass and the product thereby obtained.
2. Description of the Prior Art
The production and the use of safety glasses already belongs to the state of the art; among the safety glasses, the so-called laminated glasses have proved to be particularly advantageous, especially for use in the automobile and building industries. In said laminated glasses a film of transparent plastic material is laminated between two sheets of glass which are coupled through the action of heat and pressure.
The impact breakage of such a laminated safety glass produces no splinters. Of course, in addition to the safety requirements, said glass must also meet optical and mechanical standards. This glass must show a light transmission not lower than the light transmission of glass and, in any case, within values fixed by specific rules for the light transmission of windscreens, especially when the safety glass is used as a windscreen for cars. Moreover, it has to show high resistance to scratching and maintain constant over time its transparency, that is it must not turn yellow; the yellowing was in fact a drawback of the polyvinylacetates, polystyrenes and acetyl celluloses used in processes belonging to the state of the art.
In the present commercial trends to reduce the weight of the glasses used the double layer glasses are to be considered; so-called because they are formed of a layer of glass and of a layer of plastic material; such a layer of plastic material must show precise mechanical, physical and optical properties, for instance antitearing, self-healing properties, resistance to abrasion, resistance to hydrolysis and resistance to aging and a light transmission not lower than the light transmission of glass. The processes of production must obviously guarantee that the finished product shows all the required properties. The use in double layer glasses of plastic materials obtained through polymerization of monomers of various origin using IPN techniques (interpenetrating polymer networks) already belongs to the state of the art.
Following these techniques, appreciable improvements in the properties of the finished product have been obtained, chaining together the polymers, notwithstanding the fact that most of the polymers of different origin show an extremely limited mutual compatibility. Moreover, a process for the production of double layer glasses already belongs to the state of the art (Italian patent application number 19459A/86 filed on Feb. 19, 1986 by the same Applicant and corresponding to European patent application 87101208, publication No. 0233519) in which a particular composition essentially based on urethanic, acrylic and vinylic monomers, which can be polymerized in situ following the IPN technique, is applied through spraying and/or glazing on the glass substrate. Said glass substrate is kept in rotation at a fixed speed in order to obtain a uniform thickness of the applied composition and same composition is exposed to the action of ultraviolet and infrared radiations in order to initiate the polymerization of the monomers which will form the plastic coating. Using this process it is, however, difficult to obtain plastic coating thicknesses near to 1000 .mu.m, which are necessary to confer to the double layer composite a sufficient mechanical strength. Moreover, following the described technique, a thickness near to 1000 .mu.m can be reached by carrying out a great number of applications of the composition, which in turn is detrimental to the optical quality of the same coating. It is not possible to make a single application because the polymerization of at least two of the base components is usually initiated by UV radiation, and the problems of using these radiation to induce a complete and homogeneous polymerization in thicknesses such as the one considered in the present case are well known.